In recent years, camera modules for taking photos have begun to be incorporated in portable terminals such as mobile phones and laptop computers. Downsizing the camera modules is a prerequisite for enhancing the portability of these apparatuses. The camera module operates with an image pickup device such as a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). Recently, a pixel having the size of approximately a few micrometers has become commercially feasible, and an image pickup device with high resolution and a compact size can now be mass manufactured and marketed. This is accelerating the demand for downsizing of image pick-up lens systems so that they are able to be suitably used with miniaturized image pickup devices. It is also increasing expectations of cost reductions in image pick-up lens systems, commensurate with the lower costs enjoyed by modern image pickup devices. All in all, an image pick-up lens system needs to satisfy the oft-conflicting requirements of compactness, low cost, and excellent optical performance.
Compactness means in particular that a length from a lens edge of the lens system to an image pick-up surface should be as short as possible.
Low cost means in particular that the lens system should include as few lenses as possible; and that the lenses should be able to be formed from a resin or a plastic and be easily assembled.
Excellent optical performance can be classified into the following two main requirements:
First, a high brightness requirement, which means that the lens system should have a small F number (FNo.). Generally, the FNo. should be 2.8 or less.
Second, a high resolution requirement, which means that the lens system should appropriately correct fundamental aberrations such as spherical aberration, coma aberration, curvature of field, astigmatism, distortion, and chromatic aberration.
In a typical zoom camera, a collapsible zoom lens system is employed. In operation, the collapsible zoom lens system can extend out from the camera when powered up, and retract into the camera when powered off. However, lengths of different zoom lens systems are quite different according to the configurations thereof. For example, in a zoom lens system having a positive front group (the group on the object side with positive refracting power), optical elements with relative thicker thicknesses are employed to satisfy the requirements of predetermined zoom ratio and FNo. Therefore, even the zoom lens system retracted in the camera, more size reductions cannot be achievable.
In order to satisfy the low cost requirement, lenses used in a zoom lens system must be reduced. Among zoom lens systems, two group zoom lens systems are the simplest systems. In order to correct chromatic aberration, a two group zoom lens system employs at least four lenses is preferable.
A typical two group zoom lens system includes, from an object side to an image side, a positive lens group and a negative lens group. The two group zoom lens system can provide excellent optical performance, but the negative lens group is required to employ three or more lenses, which goes against the requirement of the low cost requirement.
Another typical two group zoom lens system includes, from an object side to an image side, a positive lens group and a negative lens group, each lens group employs two lenses. However, the length of the two group zoom lens system is relatively long, and the optical performance thereof is not ideal.
What is needed, therefore, is a zoom lens system which has compact size and excellent optical performance.